When drilling through hard surfaces such as rocks or stone, many times it is desirable to impart a reciprocating motion to the drill bit to facilitate drilling. This hammering motion of the drill bit helps break up the material while the rotating of the drill bit allows the broken up material to be removed from the hole being drilled.
A conventional hammer drill has a motor disposed in a housing, and the motor includes an armature shaft having a pinion at its end. The pinion drives a suitably arranged set of gears to rotate the output shaft. A drill chuck is mounted on the output spindle to receive a drill bit.
In conventional designs, the impact mechanism which provides the hammering action is typically associated with the face of an output gear connected to the output shaft. More specifically, a ratchet face or similar mechanism on the face of the output gear abuts a cooperating mechanism that is affixed to the drill housing. A reciprocating motion is then imparted to the drill bit when the output shaft rotates.
It is also well known in the art to provide hammer drills with the capability to switch between a conventional drilling mode, with rotation only, and a hammer drilling mode employing conventional drill rotation along with a hammer action. The hammer drill is capable of switching between the two modes, and thus eliminates the need for a separate conventional drill. An example of an adjustment mechanism for switching between conventional drilling mode and hammer drilling mode is disclosed in U.S. Pat. No. 5,447,205 assigned to the assignee of the present invention which is incorporated herein by reference.
A primary disadvantage associated with existing impact mechanisms for hammer drills is the fact that in order to accomplish a desired high blows per minute (BPM) for efficient hammer drill performance, an undesirable high output speed is required. High BPM can also be achieved by increasing the number of ramps on the impact mechanism. However, an increased number of impact ramps tends to produce a "skipping" effect and efficiency loss due to the smaller area of surface contact for each ramp.
One solution which achieves both high BPMs without a corresponding need to increase output speed is disclosed in commonly owned U.S. Pat. No. 5,653,572, and which is also incorporated herein by reference. More specifically, an intermediate gear of a two stage gear reduction arrangement is made axially displaceable and associated with a first cam mechanism for generating a reciprocating (i.e., hammer) motion. An output face is engageable with an impact face of an output gear. Engagement of the output and impact faces transmits axial displacement between the intermediate and output gears. A second cam mechanism is affixed to the housing and axially spaced from the first cam mechanism. The first and second cam mechanisms are engageable by sufficiently axially displacing the output shaft so that the output gear impact face abuts the intermediate gear output face while the first and second cam mechanisms abut each other. The first and second cam mechanisms are configured to generate reciprocating motion and cause the intermediate gear to reciprocate axially as the first cam mechanism rotates relative to the second cam mechanism, which is then transmitted to the impact face of the output gear to axially reciprocating the output shaft as it rotates.
While this arrangement satisfactorily divorces the relationship between the output shaft speed and the BPMs of the hammer action, the use of high speed motors in some drill applications, such as the high speed motors typically employed in cordless drills, requires very high reduction in speed between the drive shaft of the motor and the output shaft which rotates the chuck. A two stage gear reduction arrangement may not be suitable for such high gear reduction applications. As such, a need still exists for a hammer drill hammer mechanism which produces high BPMs without a concomitant increase in output shaft speed while also providing the ability to achieve a high gear reduction.
In addition, it is known to include a spindle locking arrangement in industrial hammer drills to prevent rotation of the output shaft while allowing the hammering action to take place. Such arrangements advantageously allow a hammer drill to operate in a third hammer only or "chipping" mode.
For example, U.S. Pat. No. 5,415,240 (Mundjar) discloses a hammer drill employing a percussion piston/striking bar hammer arrangement driven by a rotary fluid valve. Switching between a hammer, hammer/drill, and drill mode is achieved by axial movement of a pinion gear attached to the motor shaft. U.S. Pat. No. 3,955,628 (Grozinger et al) discloses a hammer drill which can be selectively switched between a hammer, hammer/drill, and drill mode by use of a cam to axially displace the output shaft to cause engagement of a hammer disk with an impact member, and a coupling member into engagement with stationary cutout. In U.S. Pat. No. 3,789,933 (Jarecki), a hammer drill is disclosed which can be selectively switched between a hammer, hammer/drill, and drill mode by use of a coupler and an axially moveable external locking collar. This arrangement acts directly on the output shaft to control rotation thereof. Finally, U.S. Pat. No. 4,236,588 (Moldan et al) and U.S. Pat. No. 4,763,733 (Neumaier) both provide hammer drills which utilize separate rotary and hammer drive mechanisms. Both arrangements also use an axially displaceable coupling sleeve to switch between rotation of the output shaft and rotation locking. Moldan '588 also discloses an intermediate mode wherein the output shaft is freely rotatable but not engaged.
While such arrangements provide hammer drills capable of operating in a hammer only mode of operation, either independent hammer and rotation drive systems are employed which undesirably increase the size, weight, and cost of the drill, or complex mechanical spindle locking arrangements are used when the hammer and rotation motions are driven by a single motor. In addition, such common drive arrangements all suffer from the inability to achieve a high BPMs without a corresponding increase in output speed, as described above.
Thus, a need exists for a hammer drill capable of operating in a third hammer only mode which utilizes a simple spindle locking arrangement, while also allowing a high BPM without a corresponding increase in output shaft speed.